Multiband orthogonal frequency division multiplexing (MB-OFDM) is a modulation technique used in some wireless communication systems such as ultra-wideband (UWB). The MB-OFDM modulation technique combines OFDM modulation with frequency hopping. It is a modulation technique suitable for devices designed to comply with Federal Communications Commission (FCC) regulations relating to UWB devices.
Unlike most other wireless systems in which the transmit power limit is typically set with respect to the total power integrated over the entire signal band, UWB devices are allowed to operate within a relatively wide frequency band provided that two criteria are met. First, the occupied bandwidth is required to meet a predefined minimum. Second, the radiated power measured over an integrating bandwidth anywhere within the signal band is required to be less than a predefined maximum. According to the current regulations, UWB devices are allowed to operate in the frequency band between 3.1 to 10.6 GHz. The occupied bandwidth is required to meet a minimum of 500 MHz and the radiated power, when measured over a bandwidth of 1 MHz anywhere within the signal band, is required to be less than −41.3 dBm.
Since in UWB the integrating bandwidth (1 MHz) is much smaller than the bandwidth of the UWB signal itself (500 MHz), the shape of the spectrum is an important issue. In order to maximize the output power of a MB-OFDM transmitter, the spectrum of the generated signal should be made as flat as possible. FIG. 1A is a diagram illustrating a frequency spectrum of an ideal UWB signal. In practice, factors such as D/A converter pulse shape, non-ideal filter characteristics, component variations and data characteristics tend to affect the shape of the spectrum. FIG. 1B is a diagram illustrating the frequency spectrum of a typical UWB signal generated by an existing device. There are peaks and variations in the frequency spectrum. The transmit power is typically limited by the largest peak in the signal spectrum. It would be desirable to have a UWB MB-OFDM transmitter design that would generate a flat output spectrum over the operating frequency range of the transmitter.